A detailed description of an interactive relationship is made between water-soluble ionic components in PM_2.5 and their precursor gases using the hourly resolution data measured at an urban monitoring site in Seoul during the year of 2010. Their diurnal variability was found to be correlated with their precursor gases (HNO₃ and NH₃). In the case of NO₃⁻ and NH₄⁺, a close similarity was seen in their diurnal trends, especially during spring. There were no noticeable differences in the formation pathway of NO₃⁻, SO₄²⁻ , and NH₄⁺ aerosols between day and night. For NO₃⁻ formation, heterogeneous hydrolysis of N₂O₅ might play a significant role in the nighttime enhancement of the oxidation rate. In addition, the concentrations of major anions (e.g. NO₃⁻, SO₄²⁻ , and Cl⁻) exhibited highly diverse seasonal patterns with their maximum values occurring in spring, summer, and winter, respectively. The overall results of this study suggest that the formation pathway of NO₃⁻ and SO₄²⁻  aerosols should be regulated by a competing relationship between the NH₃-lean (spring and winter) and NH₃-rich conditions (summer and fall).